1. Field of the Invention
The present invention relates to lasers, and more specifically, to the elimination of thermal lensing in such lasers.
2. Description of Related Art
Thermal lensing is distortion of an optical component as a result of heat, which can influence the divergence and the mode quality of a beam passing through the component. It is a problem that affects optical systems, especially high-average-power lasers, and most particularly side-pumped lasers. Zig-zag slab laser architectures have been used in some situations, in which pump light enters the slab through two opposing faces and heat is conducted out via the same or another pair of opposing faces. The laser slab's ends allow for beam entry and exit. These architectures, by virtue of the mirror symmetry of their temperature distribution and zig-zag beam propagation, give some compensation of thermal lensing. The lateral inversions that take place at the total internal reflections (TIRs) act to make the optical path length (OPL) uniform across the beam aperture because each side of the beam alternately samples a hot, high-gain and a cold, low-gain region.
In some lasers, mirror symmetry of the gain/beam propagation region is hard to achieve because the pump light is strongly absorbed, resulting in a very thin gain region. It may not be practical to fabricate a slab thin enough for two-side pumping, and cooling through the narrow, un-pumped faces may be difficult. Normally, single-sided pumping would be employed, and the un-pumped side of the slab used for heat conduction to a heat sink. In this case, the heat due to absorbed pump light enters one face and is conducted out through the opposite face, setting up a thermal gradient. One can show analytically that a beam propagating with a TIR bounce in such a medium can be arranged not to "steer" as the pump power is changed (i.e. temperature rise is varied) by having the beam bounce symmetrically off the midpoint of the pumped region. The dashed line represents the path with no pumping, and the solid line shows the refraction present with the pump turned on. The "sideways" variation of the optical path length, ##EQU1## is a measure of the "wedge," or beam steering, and vanishes at the point of symmetry, even though, ##EQU2## which relates to the thermal lensing, does not.